Re: Seat belt for middle position? Plus another ?
LATCH anchors are rated (in most vehicles) to 40lbs. Seat belts are rated to 200lbs. A tight install with seat belt is 100% as safe as a tight install with LATCH.
If you have moveable headrests, try tucking a towel or blanket under the headrest to protect the vehicle seat. It's okay to fold any extra towel under the foot part of the car seat. Verify your install is right (less than 1" movement front to back and side to side at the seat belt path) then tuck your protector down where you need it.